1. Field of the Invention
The present invention relates to a heat transfer wall for transferring heat by phase-conversion of liquid which is in contact with an outer surface of a planar plate or a heat transfer tube, and more particularly, to a heat transfer surface for use with an evaporator or a radiator.
2. Description of the Prior Art
There have been proposed various techniques concerning heat transfer walls or surfaces for enhancing boiling or evaporation heat transfer performance.
For instance, there is a method wherein an outer surface of a heat transfer wall is formed into a porous layer by sintering, weld-spraying, edging or the like. Such a heat transfer surface has a higher heat transfer performance than that of a planar and smooth surface. However, since voids in the porous layer are small, impurities contained in the boiling liquid or non-boiling liquid per se would clog the voids so that its heat transfer performance would deteriorate. Also, since the voids formed in the porous layer are made non-uniform in size, a heat transfer performance at some places are different from that at other places.
On the other hand, as shown in U.S. Pat. No. 4,060,125, there is disclosed a heat transfer wall having tunnels, openings and upper lids on a heat transfer surface. This heat transfer wall has a higher heat transfer performance. The openings are large in size in comparison with the porous layer formed by sintering. Therefore, a reduction in performance due to the clogging of impurities or non-boiling liquid may be suppressed. However, in the heat transfer wall having the opening and tunnels, there is an optimum opening diameter corresponding to a thermal load imposed on the heat transfer surface. Therefore, if the thermal load is too small or large the heat transfer performance will be lowered.
In particular, the heat transfer coefficient is lowered at a lower heat flux (for example, dw&lt;2 W/cm.sup.2 in R-11). This tendancy becomes more remarkable as a pressure is decreased, for example, to P.sub.s =0.04 MPa. Such a problem that the performance is degraded under the low heat flux and low pressure condition has been encountered also in a heat transfer surface having another porous structure (for example, metal particle sintered surface), which becomes a serious industrial problem.
On the other hand, Japanese Patent Application Laid-Open No. 14260/77 discloses a heat transfer structure in which, instead of limiting a size of the openings, by increasing a depth of the holes, the coolant is heated by the surrounding surface while passing through the passage of the holes, to be blown outside as bubbles. In such a heat transfer wall structure, since the size of the openings is not limited as shown in the specific embodiment thereof, there is no effect of replenishing the inside of the tunnels with vapor bubbles but a siphon effect obtained by the passages formed of the tunnels and the long holes is accelerated as well as the acceleration of heating and vaperization of the coolant with the long or deep holes. Accordingly, even with such a heat transfer wall structure, it is impossible to satisfactorily increase the heat transfer coefficient, in particular, under the low heat flux and the low pressure.
Also, Japanese Patent Application Laid-Open No. 45353/76 proposes a heat transfer wall characterized in that, in a boiling heat transfer surface having voids, under the outer surface, communicating with the outside through narrow openings adjacent to fins, a relationship of S.L/D.ltoreq.3 (D.ltoreq.0.12) where D (mm) is the width of the openings, L (mm) is the depth of the openings, and S (mm.sup.2) is the cross-sectional area of the voids. The outer surface of that structure has a boiling heat transfer rate twice as large as that of the smooth tube or more. However, such a proposal is related to the optimum dimensional relationship of the heat transfer surface having the continuous slit-like openings. With such a heat transfer surface, it is still impossible to solve the following problems. Namely, the location from which the bubbles through the voids and into which the liquid is supplied is not fixed and the vapor bubbles in the voids exist in an unstable fashion. Also, a great amount of liquid enters into the voids under the low heat flux and the low pressure. Thus, the heat transfer rate is extremely decreased.